This invention relates to economically attractive processes and apparatus for the alkylation of aromatic compound with mono-olefin aliphatic compound of 8 to 18 carbon atoms at low aromatic compound to mono-olefin molar ratios to provide a mono-alkylated reaction product having low co-production of heavies (dimers, polyalkylated compounds and diarylalkanes). The processes and apparatus of this invention are particularly attractive for the alkylation of benzene with linear and slightly branched olefins to provide linear alkylbenzenes and modified linear alkylbenzenes having reduced skeletal isomerization.
Alkylation of benzene produces alkylbenzenes that may find various commercial uses, e.g., alkylbenzenes can be sulfonated to produce detergents. In the alkylation process, benzene is reacted with an olefin the desired length to produce the sought alkylbenzene. The alkylation conditions comprise the presence of homogeneous or heterogeneous alkylation catalyst such as aluminum chloride, hydrogen fluoride, or zeolitic catalysts and elevated temperature.
The catalysts are not selective and other reactions of olefins can occur to produce heavies, i.e., dimers, dialkylaryl compounds and diaryl compounds. Also, skeletal isomerization of the olefin can occur, resulting in a loss of selectivity to the sought alkylbenzene. The formation of dialkylaryl compounds is particularly problematic as the reaction approaches complete conversion of the olefin and the concentration of the alkylbenzene has thus increased thereby increasing the likelihood that an olefin molecule will react with an alkylbenzene molecule rather than benzene. Accordingly, typical processes use a large excess of benzene relative to the olefin to reduce the molar ratio of the sought alkylbenzene to the olefin in the reactor. For homogeneous hydrogen fluoride catalyzed processes, the benzene to olefin ratio is generally in the range of 6:1 to 8:1. Solid catalysts are prone to generate more heavies. Hence, for these solid catalysts the mole ratio of benzene to olefin is typically greater than 15:1. For making alkylbenzenes with reduced skeletal isomerization, the benzene to olefin ratio is often in excess of 20:1 and sometimes as much as 30:1.
As the ratio of benzene to olefin increases, additional process costs are also incurred in the recovery and recycling of the unreacted benzene in the alkylation product. The refining system for alkylbenzene production is summarized in Peter R. Pujado, Linear Alkylbenzene (LAB) Manufacture, Handbook of Petroleum Refining Processes, edited by Robert A. Meyers, Second Edition, McGraw-Hill, New York, N.Y., USA, (1996), pp 1.53 to 1.66, especially pages 1.56 to 1.60. Especially for large-scale, commercial alkylation processes such as are used for the production of linear alkylbenzenes, capital and operating costs can be very important, and the addition of additional distillation steps can thus be undesirable.
A number of proposals have been made to achieve some of the benefits of high benzene to olefin feed ratios without having to incur the costs associated with using such excesses of benzene. For instance, the use of more than one reaction zone with the olefin-containing feed being introduced into each of the reactors is often done. This process has the advantage of being inexpensive from a capital and operating cost standpoint. Others have proposed processes to further improve selectivity without further increasing the molar ratio of benzene to olefins. U.S. Pat. No. 5,777,187 discloses the use of reactive distillation where benzene and the olefin are passed countercurrently though a column containing catalyst. Two problems exist with this approach. First, the capital and operating expense are increased. Second, as the catalyst needs to be regenerated or replaced, the entire reactive distillation column needs to be shut down.
Another proposal is to have a multistage reactor with product separation by distillation between the stages with the benzene and unreacted olefin passed to the subsequent reactor. However, such a process suffers from increased capital and operating costs associated with inter-stage fractionation. For example, benzene columns for removal of benzene from alkylbenzene reaction product often have at least 20 theoretical distillation trays.
One of the benefits that arises from the advent of commercially viable alkylation processes using solid alkylation catalysts is the avoidance of the use of hydrogen fluoride. As stated above, the hydrogen fluoride process, however, does benefit from being able to operate with a low benzene to olefin molar ratio, often below about 8:1, without undue production of heavies or without undue skeletal isomerization.
Accordingly processes and apparatus are sought for solid catalyst alkylation of aromatic compound with mono-olefin of 8 to 18, preferably 8 to 16, carbon atoms per molecule which can use lower aromatic compound to olefin molar ratios without undue production of heavies, especially if such processes do not result in undue skeletal isomerization. Also desired are solid catalyst alkylation processes and apparatus that can retrofit a hydrogen fluoride catalyst alkylation unit without the need to replace any of the reaction product refining system, especially the benzene distillation column. Consequently, the retrofit must be capable of providing an alkylation product of sought yields using an aromatic compound to olefin molar ratio of about 10:1 or less.